After a power down, have you already tried removing all external media devices and re-Coldbooting? If, though remotely possible, there is an imminent HD "logical" or physical failure approaching, then consider something many DR specialists recommend:
If affordable, if possible, via a usb or dvd boot, clone the problematic source HD onto any trusted reliable target HD, store target HD in a safe place, because, if the worst case arrives, you have whatever that was salvaged. Your backups will have to fill in the rest.